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Apple has again tweaked its iOS App Store algorithms, and the changes have brought about widespread search ranking changes across both iPhone and iPad devices. That’s search rankings, to be clear – meaning where an app is returned when a user searches for a particular keyword, like “music” or “banking,” for example, in the App Store. In other words, it’s not “ranking,” as in where an app lies on the App Store’s top charts.

The change was first spotted by app marketing tools provider MobileDevHQ, which keeps a close eye on factors that would affect App Store Optimization, or ASO as it’s often called. (Basically, that’s SEO for mobile app store search).

The company says the change occurred between 12/11 and 12/12, and is affecting both iPhone and iPad applications. The average ranking change they saw was 41.5 positions, which is 8 times the normal ranking change seen on an average day for iPhone results. Typically, the usual daily change is around 5 positions, for comparison’s sake.

The search ranking changes on the iPhone seem to be larger than those seen during the iOS 7 release (see bar chart below), which was also significant. And it’s affecting mainly App Store search rankings, as noted above, not the top charts, though this is now under investigation.

Some examples (see charts below) show the impact this has had on various popular apps for search terms including “banking,” “home,” “music,” and “travel.” Here you’ll see that some apps have moved only a little, while others quite a bit. For example, Pandora went from 8 to 5, following the adjustment, while Rdio moved up from 96 to 53. Meanwhile, under travel, Hipmunk jumped up from 58 to 34, and Hotwire went from 116 to 66.

The charts have further examples, and these charts themselves are just a small sampling of MobileDevHQ’s tests, which involved 30,000 keyword searches.

As the company further dug into the data related the keyword rankings, they also found that:

Per keyword, 62.8% of apps saw a ranking change of at least 5 positions. (Normally about 17% of apps for a given keyword see a ranking change of at least 5 positions per day.)

Per keyword, 40% of apps saw a ranking change of at least 20 positions. (Normally about 3% see a ranking change of that magnitude per day.)

And :

Looking at the top 5 apps for any keyword, 31% of them changed. (Normally only about 8% change day-to-day.)

Of course, Apple continually makes adjustments to its App Store algorithms, but it’s not as common for it to make a large-scale change that move apps around this much in terms of App Store search results.

However, we’ve seen Apple focus on making its App Store search better in recent weeks – for example, in November we confirmed that the App Store’s search engine had begun correcting for misspellings and other “fat finger” typos and mistakes. And in the past, we’ve also seen Apple adjust for ratings, in addition to just download volumes and velocity, for instance.

What we don’t know right now is why Apple is doing this – is it part of a larger test on its part to shift how a weighting factor works in its search rankings? Is it in response to a particular problem that Apple is now trying to correct for, as has been common in the past? (Apple’s algorithms often change when publishers learn how to game its results.)

By spot checking the results over time, it appears that the changes have so far been sustained, MobileDevHQ reports, which would point to a more permanent algorithm change, and not a test.